1. Field of the Invention
The present invention relates to a motor driving apparatus that includes a position detecting unit.
2. Description of the Related Art
Stepping motors have such characteristics as its small size, high torque and long life, thus easily realizing digital position control operations under the open loop control. Hence, the stepping motors have widely been used for information home appliances, such as cameras and optical disk devices, and for business appliances, such as printers and projectors.
However, when a stepping motor performs high-speed operation or performs operation under heavy load, problems arise in that the stepping motor may step out, and the driving efficiency of the stepping motor is lower than that of a brushless motor or DC motor.
In order to solve the problems, there is provided a conventionally known technique for preventing the stepping out of the motor. According to this technique, an encoder is mounted on the stepping motor, and the electricity is switched on/off according to the position of the rotor, thus accomplishing an operation of the so-called brushless DC motor.
Japanese Patent Publication No. 06-067259 and Japanese Patent Application Laid-Open No. 2002-359997 discuss such motors. According to these techniques, the rise delay of current is compensated for so as to enable high-speed operations, by advancing the phase of a signal obtained by a non-contact type sensor mounted in the motor according to the speed and also switching the current to flow through the coil according to the signal.
In the motors discussed in the above patent documents, it is necessary to mount two magnetic sensors with high accuracy. The necessity will now be described.
FIG. 11 illustrates the torque on a rotor when a constant current flows through coils. Because currents can flow through the two coils respectively in the forward and backward directions, four types of torque distributions can be generated as illustrated in FIG. 11. These distributions have the same wave form substantially in a sine wave form and have a phase difference of 90° in electrical angle.
The electrical angle expresses one cycle of the sine wave at 360°. When the number of poles of the rotor is n, the actual angle is (2×electrical angle)/n.
When the motor is driven to rotate, the electricity is sequentially switched on to flow through the coils, thereby always obtaining the high torque having a torque waveform T1 illustrated in FIG. 11. At this time, the electricity is switched on to flow through the coils at a timing determined based on a signal from a magnetic sensor. If two magnetic sensors are arranged with a space at 90° electrical angle from each other, the electricity can be switched at the most efficient timing.
However, if there is an error in the set position of the magnetic sensor, the torque waveform is deteriorated as illustrated by T2 in FIG. 11, thus decreasing the motor efficiency.
Therefore, it is necessary to perform a process for correcting the set position of the magnetic sensor at the assembling of the motor, resulting in increasing the manufacture cost and decreasing the motor quality.
Japanese Patent No. 3621696 solves the above-mentioned problem as follows. Two magnetically sensitive poles are provided in one chip, which is a magnetic sensor having an opening. An angle between straight lines connecting the centers of the magnetically sensitive poles to the center of the rotary shaft is half (½) of an angle between the straight lines connecting the centers of adjacent magnetic poles of a magnet to the center of the rotary shaft.
With this configuration, the two magnetically sensitive poles can be arranged with a space based on the component level. In addition, the electricity is switched on to flow through the coils at an optimum timing so as to avoid the torque down of the motor.
Signals output from the two magnetically sensitive poles are generated to have a 90° phase difference from each other. Thus, the timing to switch the electricity to flow through the coils is controlled according to whether the signals are positive or negative, thus realizing the motor with high efficiency.
The position of the magnetic sensor is determined based on the opening thereof, thus enabling setting the position of the magnetic sensor with high accuracy. However, the method discussed in Japanese Patent No. 3621696 has a problem in that the degree of freedom of setting the magnetic sensor decreases. This problem will now be described.
In order to increase the efficiency of the motor as described above, it is necessary that a current flowing through the first coil has a phase difference of 90° in electrical angle from a current flowing through the second coil. Thus, according to the technique discussed in Japanese Patent No. 3621696, an angle between the straight lines connecting the centers of the magnetically sensitive poles to the center of the rotary shaft is half (½) of an angle between the straight lines connecting the centers of adjacent magnetic poles of the magnet to the center of the rotary shaft.
More specifically, once the number of poles of the motor and the positions of the sensors in radial direction are determined, the distance between the two sensors is determined. Therefore, it is necessary to prepare individual special sensors according to the number of poles of the motor or the size of the magnet, thus complicating the reduction of the cost of the sensors.